The image display device can be incorporated as a headup display device in various kinds of an operable vehicle such as automobile, train, ship and vessel, helicopter, airplane.
Japanese Patent Application Publication No. 2009-128659 (Reference 1) and No. 2010-145745 (Reference 2) disclose a headup display device as an image display device which two-dimensionally scans alight beam to display an image.
This device includes a deflector to two-dimensionally deflect a light beam modulated in intensity by an image signal and scans a micro lens array with the deflected light beam to form an image thereon. The image is enlarged by a virtual image optical system and formed as an enlarged virtual image.
A reflective element is provided prior to the position of the enlarged virtual image to reflect the image to an observation side for observation.
A laser beam with high optical energy density and directivity is suitable for the light beam forming the image, as described in References 1 and 2.
However, due to the coherence of a laser beam, interfering noise such as speckle is likely to occur in an observed image. Interference fringes are a typical example of interfering noise. Interference fringes bring about degradation of image quality and visibility.
Reference 1 discloses an interfering noise removing method. Therein, micro convex cylindrical lenses are arranged as a micro lens array and the beam diameter of a scanning coherent light beam is set to a smaller value than a pitch with which the micro lenses are arranged. Then, a light source is configured to emit a light beam in pulse in synchronization with scanning so that the light beam is irradiated not astride border portions of neighboring micro lenses but only on the micro lenses.
Alternatively, an optical shield layer can be provided on the border portions in order to block the light beam from irradiating the border portions.
Interfering noise can be effectively removed in the above manners. However, to emit the light beam in pulse in synchronization with scanning and illuminate only the micro lenses, a light source and a portion scanning the light beam need to have a complex structure.
Further, with the shield layer formed at the borders, a light beam can be continuously scanned, however, the light blocking by the shield layer may result in a decrease in the brightness of a displayed enlarged virtual image.